1. Field of the Invention
The present invention relates to a reflecting device for electromagnetic waves, comprising a mirror carrier made of a material with at least approximately vanishing thermal expansion and at least one reflective layer applied on said mirror carrier.
Moreover, the invention relates to a method for producing such a device for reflecting electromagnetic waves.
More specifically, the invention relates to electromagnetic waves in a wavelength range less than 200 nm.
2. Description of the Related Art
DE 198 30 449 A1 discloses a mirror substrate which comprises crystal and is provided with an amorphous layer of the order of magnitude of 1 to 100 μm, which allows a much higher polishing quality than the mirror substrate itself. In this case, the use of such a mirror is provided in EUV projection exposure installations.
In general, it is known that mirrors in EUV installations, for example EUV lithography systems, must have a very good figure, which means that the errors in the low spatial frequency range in EUV (spatial wavelengths≧1 mm) are small. Furthermore, such mirrors must have small roughnesses in the mid spatial frequency range (MSFR: mid spatial frequency roughness; at EUV spatial wavelengths typically between 1 μm−1 and 1 mm−1. It is furthermore known that part of the incident light is absorbed by the multilayer reflection layers that are customary and known per se, the so-called Distributed Bragg Reflectors (DBR), and converted into heat.
To ensure that the surface form of the mirror remains stable during operation under these thermal loads, it is necessary to use a material with the smallest possible thermal expansion coefficient as carrier material for such mirrors. In particular, mention shall be made here of glass-ceramic materials which are composed of a plurality of components having different thermal expansion coefficients, so that the material has macroscopically no or a vanishing thermal expansion coefficient. The materials sold under the proprietary names ZERODUR® or Clear Ceram® are applicable as an example of said material.
In addition to these requirements that are to be satisfied in the region of such mirrors, a very small roughness of approximately 0.1 nm rms of the surface must additionally be ensured in the region of high spatial frequencies (HSFR: high spatial frequency roughness; in EUV: spatial wavelengths≦1 μm), in order to ensure a corresponding reflectivity in the EUV region of the multilayer to be applied to the mirror surface.
According to the current prior art, the required HSFR in the region of approximately 0.1 nm rms can be achieved by means of superpolishing methods on various materials, such as quartz glass, ULE, silicon, or else on glass ceramics, such as ZERODUR® or Clear Ceram®. However, these superpolishing methods have the disadvantage that, at least when aspherical mirrors are used, generally the figures and, under certain circumstances, also the longer-wave MSFR components are impaired again, so that the superpolishing methods have to be followed by a fine correction process.
Particularly when using glass-ceramic materials, such as ZERODUR®, in which crystallites having a corresponding thermal expansion coefficient are embedded in an amorphous matrix having a different thermal expansion coefficient, a fine correction method, in particular based on ion beam figuring, leads to a serious impairment of the HSFR.